One prior art invention which relates to the present invention is described in U.S. Pat. No. 4,555,662, issued Nov., 1985, this patent disclosing a quantitative measurement method for inclusions, the method now being generally referred to as Liquid Metal Cleanliness Analysis (LiMCA for short). The LiMCA method and apparatus were originally developed for detecting nonmetallic inclusions during aluminium refining, but its application to iron and steel refining has also been investigated.
The LiMCA method is sometimes also referred to as the Electric Sensing Zone method (ESZ for short), the principle of the method being that when such an inclusion entrained in an electrically conductive fluid passes through an electrically-insulated orifice the electrical resistance of the fluid which is flowing through the orifice changes in proportion to the volume of the particle. The instantaneous change in the resistance is detected as a pulse in electrical potential between two electrodes on opposite sides of the orifice, and the number and size of the particles can be directly measured in the following manner.
First, if the particles are assumed to be spherical and of diameter d and the orifice is assumed to be cylindrical of diameter D, then the change R in the electrical resistance when a particle passes through the orifice is given by the following equation: EQU .DELTA.R=(4.rho.d.sup.3)/(.pi.D.sup.4) (1)
Where .rho. is the electrical resistivity of the fluid.
In actual practice, Equation (1) must be corrected by a correction factor F(d/D), which is given by the following equation: EQU F(d/D)=[1-0.8(d/D).sup.3 ].sup.-1 ( 2)
Thus, .DELTA.R is actually expressed by the following equation: EQU .DELTA.R=((4.rho.d.sup.3)/(.pi.D.sup.4).times.[1-0.8(d/D).sup.3 ].sup.-1( 3)
If the electric current through the orifice is I, then the pulse .DELTA.V is the electric potential when a particle of diameter d passes through the orifice is given by the following equation: EQU .DELTA.V=I(.DELTA.R) (4)
A previously-disclosed inclusion sensor probe which applies the above-described principles and for use with molten metal comprises a hollow inner first electrode made from an electrically-conducting, heat-resistant material, this inner electrode being supported inside a quartz tube and connected to an electrode rod through a graphite reinforcing member. An orifice is provided in a portion of the quartz tube near to its lower end, while a cylindrical layer which protects against slag is disposed around a central portion of the outside surface of the quartz tube. The tube is mounted on a water-cooled support apparatus through a coupler which is equipped with an O-ring to seal the joint between the tube and the coupler. The necessary outer second electrode consists of a rod separate from the probe and extending close to the orifice.
When a measurement is to be performed the inside of the hollow electrode, which serves as a chamber to receive the molten metal, is evacuated and the molten metal is sucked inside through the orifice. At this time, the change in electric resistance between the inner and outer electrodes is measured and amplified by conventional means, and the sizes and number of inclusions are determined.
The above-described sensor probe and others are used to perform continuous measurement by the LiMCA method in order to detect inclusions in molten aluminium and determine particle size distributions. Molten aluminium has a relatively low melting temperature of about 700.degree. C., so there are a number of different materials available from which the probe and the electrodes can be made. However, the working temperatures of molten metal baths of metals like iron and titanium are much higher than for aluminium (above 1550.degree. C.), and at such temperatures there are considerable problems with lack of resistance of the probe and the electrodes to heat, so that it is difficult to employ these known sensors.
The known probes for inclusion sensors as used hitherto for high melting point metals are all of the continuous measurement type, i.e. once the probe is immersed in the molten metal measurements are performed for a period of at least 30-40 minutes while being cooled with water, and usually this is the maximum useful life of such a probe. The known continuous measurement probes for inclusion sensors have the advantage that they can continuously monitor changes in the level of inclusions in molten metal, but have the following drawbacks.
(1) When used in a bath of high melting point molten metal, such as iron or steel, then in order to guarantee sufficient resistance to heat and reaction the body of probe must be made of a high-quality material, such as quartz, as a result of which the probe is expensive.
(2) When used with these high melting point metals, in spite of the fact that each probe is made of expensive materials, the probe still has life span of only 30-40 minutes, despite the use of water cooling.
(3) In order to provide sufficient heat resistance the probe support mechanism must be equipped with a water cooling mechanism. As a result, the probe and the probe support mechanism end up being large and difficult to handle, and a mechanism for raising and lowering the probe also must be large, so that equipment costs are high.